6 



had the greatest pleasure in examining the original base apparatus of Bessel in 

 the Prussian geodetic office. The Brunner apparatus aims at the simplicity of 

 the Borda thermometer, and is therefore composed of two four-meter bars of 

 different metals, having largely different co-efficients of expansion. The lower 

 bar is platinum; immediately above it is the brass bar of the same cross sec- 

 tion. At one extremity of the compound, or rather composite bar, the two bars 

 are joined, but are free towards the other extremity. At the free ends the 

 upper bar has a slot in it that receives a piece soldered to the lower bar, 

 whereby the upper surface of this piece is flush with the upper surface of the 

 brass bar. Upon one is cut a series of graduated lines; upon the other a 

 vernier. "When the bars expand or contract through increase or decrease of 

 temperature they do so unequally, and the difference of that change is read and 

 is determined by the scale and vernier. 



The base bars of Bessel are four in number, and each composite bar is 

 formed of two bars of different metals of different rates of expansion for equal 

 increments of heat. In this case the lower bar is iron and the upper is zinc. 

 One end of each system is formed by the junction of the two bars, and thence 

 the bars are free. Instead of a scal-» upon one and a vernier upon the other, 

 the zinc bar terminates close to a small projection upon the upper surface of 

 the iron bar, and the difference of expansion was measured by the insertion of 

 long graduated wedges of glass in the space between the end of the zinc bar and 

 the iron stud. 



Either of these forms of apparatus has great merit in the extreme simplicity 

 of the form and fewness of parts, and it appears to me that with thorough skill 

 in the observer, accuracy of comparison with the given standard bar, simplicity 

 and stability of the supporta, and the proper means of effecting the contacts, 

 the method is susceptible of great accuracy. But I was very far from being 

 satisfied with the character of the supports and the methods and appliances of 

 measurement. These, however, I discuss fully in my official report to the Su- 

 perintendent of the Coast Survey. 



Without detaining the attention of the Academy any longer, I may mention, 

 in brief, the general conclusion to which I arrived. While I saw much of deep 

 interest, there was no single instrument that 1 would unreservedly recom- 

 mend for adoption in its entirety. What I principally learned was really wJiat 

 not to copy. And whilst awarding high credit to the instrument makers of 

 Europe, and keenly alive to their courtesy, I am convinced that we do not 

 need to go to Europe for geodetic instruments, although we should possess 

 copies of their finest efforts for comparison with our own. With our own ob- 

 servers and mechanicians working in harmony and striving to attain the highest 

 excellence, I firmly believe that we are fully competent to lead in this scien- 

 tific effort. Both parties fully appreciate the fundamental requisites to success, 

 and the character of the geodetic work of the United States demands that the 

 mechanical means shall be adequate to the delicate manipulation and require- 

 ments of the most skillful observers. 



